Fellow TVS / VMP Owners - What RPM do you see full boost at under WOT?

[Uncle Rusty]

Hey guys, I was just wondering when launching at WOT from a lower RPM, at what point do you guys see full boost at? I'm currently running a VMP Stage 2 w/ 82mm pulley which gets up to about 9.5-10psi here close to sea level, and in most cases I'm not seeing the 9-10psi until around 3000-3500 RPM. The car feels sluggish in the 1000-3000 band before taking off like a rocket, so was just wondering if that's normal or not?

I realize that a lot of it has to do with the tune, and I've been working with Justin on it and am going to get some more datalogs, but my WOT pulls look really good from what he said. Those of you with manuals, if you put the car in 5th at a low speed and go WOT, how long does it take you to hit full boost? Before 3-3.5k mine is stuck at around 5psi.

 

[by9468840]

keep on going wot in 5th from low rpm and you won't soon have an engine to put boost in.
Just don't get too hung up on how psi builds. Justin knows his shit. Trust him.

 

[Uncle Rusty]

Any reason it particular why that's bad for the engine? Too much stress on the internals under low load?

I definitely trust him lol, I just want to make sure this is normal boost behavior with the TVS. What I don't trust is my mechanical roush boost gauge... I've had issues with mechanical boost gauges on past turbo cars being the primary source of a boost leak.

 

[Scott8583]

I don't have a manual trans, but going WOT at low speed in 5th gear sounds like a bent rod waiting to happen... And possibly spark knock.

I'd wouldn't worry so much about your boost gauge. Rely more on data logs, actually data logs at a sticky track are best. I don't run a boost gauge. I know mine made 14psi on the dyno, if the power drops I'll worry about a gauge then...

 

[BrianH87]

Remember what it was like trying to pedal a 10 speed bike in high gear, from a slow speed? It felt like your legs were gonna break off. That is what your rods feel like at low rpms.

 

[Uncle Rusty]

That makes perfect sense. It's not something I normally do when driving a car, but I did end up doing it a few times while still getting used to the new power levels of the car. I'll make sure to avoid bogging the engine like that in the future though. The data logs look good so I'll just keep my mind on those.

 

[Riptide]

One thing about boost and altitude. Only commenting since you referenced sea level.

A car with a 10lb. pulley will see the same boost on the gauge at 5000 feet as a car at sea level. The supercharger still produces the same amount of boost. What you have at higher altitudes is less cylinder pressure and correspondingly less power output.

psia vs. psig

This is why cars at higher altitudes can run smaller pulleys, make more boost, and end up producing the same whp as the same car at sea level.

 

[Zona14Stang]

One thing about boost and altitude. Only commenting since you referenced sea level.

A car with a 10lb. pulley will see the same boost on the gauge at 5000 feet as a car at sea level. The supercharger still produces the same amount of boost. What you have at higher altitudes is less cylinder pressure and correspondingly less power output.

psia vs. psig

This is why cars at higher altitudes can run smaller pulleys, make more boost, and end up producing the same whp as the same car at sea level.

I had a 82 mm pulley here is AZ at about 1600ft eleveation and never saw more than 9 psi, upgraded to the 79 mm and now I am seeing 10 psi

 

[stangn]

Absolute and gauge are the same reading, just in a different form.
14.7 psia = 0 psig.
There for 10 psig = 24.7 psia.
My useless input for the day lol

 

[Riptide]

I had a 82 mm pulley here is AZ at about 1600ft eleveation and never saw more than 9 psi, upgraded to the 79 mm and now I am seeing 10 psi

My friend with a centri gets the exact same psig he is supposed to with the pulley he has on there as the same car at sea level. He is at 4000+ feet.

The 79mm pulley should produce 10-11psi at 4000' just like it does at sea level. I'll have that system set up in a month and can test that setup.

http://www.rx8club.com/series-i-maj...fect-altitude-superchargers-vs-turbos-186241/
^^ Explained fairly well there.

My 3v KB car produced 8.5 PSI with the stock pulley at 5000' altitude. Just like it would at sea level.
http://www.kennebell.net/KBWebsite/FAQ_pg/layouts/faq-answers6.htm

How much boost do I lose per 1000' elevation? None. Our supercharger always produces it's rated boost over atmospheric pressure. Approx. .5 psi per 1000' of atmospheric pressure is lost due to elevation. Boost plus atmospheric pressure is called absolute pressure. 14.7 psi at sea level +5 psi boost = 19.7 absolute psi.

 

[Uncle Rusty]

Right, that's always good to keep in mind. I do have a desire to move out to Colorado in the next few years so the smaller pulley would make sense in that case without being dangerous to the stock internals.